ATX-101, a Peptide Targeting PCNA, Has Antitumor Efficacy Alone or in Combination with Radiotherapy in Murine Models of Human Glioblastoma

Targeting the "Undruggable": Small-Molecule Inhibitors of Proliferating Cell Nuclear Antigen (PCNA) in the Spotlight in Cancer Therapy

PCNA plays multiple roles in cancer development, including cell proliferation regulation, DNA repair, replication, and serving as a widely used biomarker and therapeutic target. Despite its significant role in oncology, PCNA has historically been considered "undruggable" due to the absence of known endogenous small molecule modulators and identifiable ligand binding sites. Unlike other protein-protein interfaces, PCNA lacks explicit binding grooves, featuring a relatively small and shallow surface pocket, which hinders the discovery of traditional small molecule targets. Recent breakthroughs have introduced promising PCNA-targeting candidates, with ATX-101 and AOH1996 entering phase I clinical trials for cancer therapy, garnering academic and industry interest. These achievements provide new evidence for PCNA as a drug target. This article provides insight and perspective on the application of small-molecule PCNA inhibitors in cancer treatment, covering PCNA function, its relationship with cancer, structural modification of small molecule inhibitors, and discovery strategies.

Exploring the Chemical Features and Biomedical Relevance of Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) are a diverse group of peptides, typically composed of 4 to 40 amino acids, known for their unique ability to transport a wide range of substances-such as small molecules, plasmid DNA, small interfering RNA, proteins, viruses, and nanoparticles-across cellular membranes while preserving the integrity of the cargo. CPPs exhibit passive and non-selective behavior, often requiring functionalization or chemical modification to enhance their specificity and efficacy. The precise mechanisms governing the cellular uptake of CPPs remain ambiguous; however, electrostatic interactions between positively charged amino acids and negatively charged glycosaminoglycans on the membrane, particularly heparan sulfate proteoglycans, are considered the initial crucial step for CPP uptake. Clinical trials have highlighted the potential of CPPs in diagnosing and treating various diseases, including cancer, central nervous system disorders, eye disorders, and diabetes. This review provides a comprehensive overview of CPP classifications, potential applications, transduction mechanisms, and the most relevant algorithms to improve the accuracy and reliability of predictions in CPP development.

Cancer-Targeting Applications of Cell-Penetrating Peptides

Cell-penetrating peptides (CPPs) offer a unique and efficient mechanism for delivering therapeutic agents directly into cancer cells. These peptides can traverse cellular membranes, overcoming one of the critical barriers in drug delivery systems. In this review, we explore recent advancements in the application of CPPs for cancer treatment, focusing on mechanisms, delivery strategies, and clinical potential. The review highlights the use of CPP-drug conjugates, CPP-based vaccines, and their role in targeting and inhibiting tumor growth.

Multiple Myeloma Cells with Increased Proteasomal and ER Stress Are Hypersensitive to ATX-101, an Experimental Peptide Drug Targeting PCNA

Objectives: To examine the regulatory role of PCNA in MM, we have targeted PCNA with the experimental drug ATX-101 in three commercial cell lines (JJN3, RPMI 1660, AMO) and seven in-house patient-derived cell lines with a more primary cell-like phenotype (TK9, 10, 12, 13, 14, 16 and 18) and measured the systemic molecular effects. Methods: We have used a multi-omics untargeted approach, measuring the gene expression (transcriptomics), a subproteomics approach measuring mainly signalling proteins and proteins in complex with these (signallomics) and quantitative metabolomics. These results are supplemented with traditional analysis, e.g., viability, Western and ELISA analysis. Results: The sensitivity of the cell lines to ATX-101 varied, including between three cell lines derived from the same patient at different times of disease. A trend towards increased sensitivity to ATX-101 during disease progression was detected. Although with different sensitivities, ATX-101 treatment resulted in numerous changes in signalling and metabolite pools in all cell lines. Transcriptomics and signallomics analysis of the TK cell lines revealed that elevated endogenous expression of ribosomal genes, elevated proteasomal and endoplasmic reticulum (ER) stress and low endogenous levels of NAD+ and NADH were associated with ATX-101 hypersensitivity. ATX-101 treatment further enhanced the ER stress, reduced primary metabolism and reduced the levels of the redox pair GSH/GSSG in sensitive cells. Signallome analysis suggested that eleven proteins (TPD52, TNFRS17/BCMA, LILRB4/ILT3, TSG101, ZNRF2, UPF3B, FADS2, C11orf38/SMAP, CGREF1, GAA, COG4) were activated only in the sensitive MM cell lines (TK13, 14 and 16 and JJN3), and not in nine other cancer cell lines or in primary monocytes. These proteins may therefore be biomarkers of cells with activated proteasomal and ER stress even though the gene expression levels of these proteins were not elevated. Interestingly, carfilzomib-resistant cells were at least as sensitive to ATX-101 as the wild-type cells, suggesting both low cross-resistance between ATX-101 and proteasome inhibitors and elevated proteasomal stress in carfilzomib-resistant cells. Conclusions: Our multi-omics approach revealed a vital role of PCNA in regulation of proteasomal and ER stress in MM.

Glioblastoma stem cell metabolism and immunity

Despite enormous efforts being invested in the development of novel therapies for brain malignancies, there remains a dire need for effective treatments, particularly for pediatric glioblastomas. Their poor prognosis has been attributed to the fact that conventional therapies target tumoral cells, but not glioblastoma stem cells (GSCs). GSCs are characterized by self-renewal, tumorigenicity, poor differentiation, and resistance to therapy. These characteristics represent the fundamental tools needed to recapitulate the tumor and result in a relapse. The mechanisms by which GSCs alter metabolic cues and escape elimination by immune cells are discussed in this article, along with potential strategies to harness effector immune cells against GSCs. As cellular immunotherapy is making significant advances in a variety of cancers, leveraging this underexplored reservoir may result in significant improvements in the treatment options for brain malignancies.

Correction: Gravina et al. ATX-101, a Peptide Targeting PCNA, Has Antitumor Efficacy Alone or in Combination with Radiotherapy in Murine Models of Human Glioblastoma. Cancers 2022, 14, 289

In the original publication [...].

Progress of nanoparticle drug delivery system for the treatment of glioma

Gliomas are typical malignant brain tumours affecting a wide population worldwide. Operation, as the common treatment for gliomas, is always accompanied by postoperative drug chemotherapy, but cannot cure patients. The main challenges are chemotherapeutic drugs have low blood-brain barrier passage rate and a lot of serious adverse effects, meanwhile, they have difficulty targeting glioma issues. Nowadays, the emergence of nanoparticles (NPs) drug delivery systems (NDDS) has provided a new promising approach for the treatment of gliomas owing to their excellent biodegradability, high stability, good biocompatibility, low toxicity, and minimal adverse effects. Herein, we reviewed the types and delivery mechanisms of NPs currently used in gliomas, including passive and active brain targeting drug delivery. In particular, we primarily focused on various hopeful types of NPs (such as liposome, chitosan, ferritin, graphene oxide, silica nanoparticle, nanogel, neutrophil, and adeno-associated virus), and discussed their advantages, disadvantages, and progress in preclinical trials. Moreover, we outlined the clinical trials of NPs applied in gliomas. According to this review, we provide an outlook of the prospects of NDDS for treating gliomas and summarise some methods that can enhance the targeting specificity and safety of NPs, like surface modification and conjugating ligands and peptides. Although there are still some limitations of these NPs, NDDS will offer the potential for curing glioma patients.

Targeting proliferating cell nuclear antigen (PCNA) for cancer therapy

Proliferating cell nuclear antigen (PCNA) is an essential scaffold protein in many cellular processes. It is best known for its role as a DNA sliding clamp and processivity factor during DNA replication, which has been extensively reviewed by others. However, the importance of PCNA extends beyond its DNA-associated functions in DNA replication, chromatin remodelling, DNA repair and DNA damage tolerance (DDT), as new non-canonical roles of PCNA in the cytosol have recently been identified. These include roles in the regulation of immune evasion, apoptosis, metabolism, and cellular signalling. The diverse roles of PCNA are largely mediated by its myriad protein interactions, and its centrality to cellular processes makes PCNA a valid therapeutic anticancer target. PCNA is expressed in all cells and plays an essential role in normal cellular homeostasis; therefore, the main challenge in targeting PCNA is to selectively kill cancer cells while avoiding unacceptable toxicity to healthy cells. This chapter focuses on the stress-related roles of PCNA, and how targeting these PCNA roles can be exploited in cancer therapy.

Identification of a potent PCNA-p15-interaction inhibitor by autodisplay-based peptide library screening

Proliferating cell nuclear antigen (PCNA) is an essential factor for DNA metabolism. The influence of PCNA on DNA replication and repair, combined with the high expression rate of PCNA in various tumours renders PCNA a promising target for cancer therapy. In this context, an autodisplay-based screening method was developed to identify peptidic PCNA interaction inhibitors. A 12-mer randomized peptide library consisting of 2.54 × 106 colony-forming units was constructed and displayed at the surface of Escherichia coli BL21 (DE3) cells by autodisplay. Cells exhibiting an enhanced binding to fluorescent mScarlet-I-PCNA were enriched in four sorting rounds by flow cytometry. This led to the discovery of five peptide variants with affinity to mScarlet-I-PCNA. Among these, P3 (TCPLRWITHDHP) exhibited the highest binding signal. Subsequent flow cytometric analysis revealed a dissociation constant of 0.62 μM for PCNA-P3 interaction. Furthermore, the inhibition of PCNA interactions was investigated using p15, a PIP-box containing protein involved in DNA replication and repair. P3 inhibited the PCNA-p1551-70 interaction with a half maximal inhibitory activity of 16.2 μM, characterizing P3 as a potent inhibitor of the PCNA-p15 interaction.

Radiotherapy of high-grade gliomas: dealing with a stalemate

This article discusses the studies on radiotherapy of high-grade gliomas published between January 1, 2022, and June 30, 2022, with special reference to their molecular biology basis. The focus was on advances in radioresistance, radiosensitization and the toxicity of radiotherapy treatments. In the first half of 2022, several important advances have been made in understanding resistance mechanisms in high-grade gliomas. Furthermore, the development of several radiosensitization procedures for these deadly tumors, including studies with small molecule radiosensitizers, new fractionation protocols, and new immunostimulatory agents, has progressed in both the preclinical and clinical settings, reflecting the frantic research effort in the field. However, since 2005 our research efforts fail to produce significant improvements to treatment guidelines for high-grade gliomas. Possible reasons for this stalemate and measures to overcome it are discussed.

Treatment of De-Differentiated Liposarcoma in the Era of Immunotherapy

Well-differentiated/de-differentiated liposarcoma (WDLPS/DDLPS) is one of the most common histologic subtypes of soft tissue sarcoma (STS); however, treatment options remain limited. WDLPS and DDLPS both exhibit the characteristic amplification of chromosome region 12q13-15, which contains the genes CDK4 and MDM2. DDLPS exhibits higher amplification ratios of these two and carries additional genomic lesions, including the amplification of chromosome region 1p32 and chromosome region 6q23, which may explain the more aggressive biology of DDLPS. WDLPS does not respond to systemic chemotherapy and is primarily managed with local therapy, including multiple resections and debulking procedures whenever clinically feasible. In contrast, DDLPS can respond to chemotherapy drugs and drug combinations, including doxorubicin (or doxorubicin in combination with ifosfamide), gemcitabine (or gemcitabine in combination with docetaxel), trabectedin, eribulin, and pazopanib. However, the response rate is generally low, and the response duration is usually short. This review highlights the clinical trials with developmental therapeutics that have been completed or are ongoing, including CDK4/6 inhibitors, MDM2 inhibitors, and immune checkpoint inhibitors. This review will also discuss the current landscape in assessing biomarkers for identifying tumors sensitive to immune checkpoint inhibitors.

Hypocretin-1 suppresses malignant progression of glioblastoma cells through Notch1 signaling pathway

Hypocretin-1 is a multifunctional neuropeptide that has been identified as a potential antitumor agent for its role in inhibiting tumor growth, including in colon cancer, neuroendocrine tumor, and prostate cancer. However, the role and mechanism of hypocretin-1 in the occurrence and development of malignant glioma have not been well studied. Therefore, we investigated the effect of hypocretin-1 on glioblastoma proliferation, apoptosis, migration and invasion and its mechanism. We found that the hypocretin-1 receptor was expressed in both glioma cell lines and glioma tissues. Hypocretin-1 treatment can inhibit glioblastoma cell proliferation, migration and invasion, and induce cell apoptosis. Meanwhile, hypocretin-1 treatment significantly reduces tumor growth rate and tumor weight. In addition, mechanistic studies have found that hypocretin-1 exerts antitumor effects by inhibiting NOTCH signaling pathway. Overexpression of NICD significantly reversed the antitumor effect of hypocretin on glioblastoma. Taken together, these findings suggest that hypocretin-1 inhibits glioblastoma proliferation, migration and invasion and induces apoptosis in vitro and in vivo through NOTCH signaling pathway.

MALDI Imaging Mass Spectrometry of High-Grade Gliomas: A Review of Recent Progress and Future Perspective

Glioblastoma (GBM) is the most common malignancy of the brain with a relatively short median survival and high mortality. Advanced age, high socioeconomic status, exposure to ionizing radiation, and other factors have been correlated with an increased incidence of GBM, while female sex hormones, history of allergies, and frequent use of specific drugs might exert protective effects against this disease. However, none of these explain the pathogenesis of GBM. The most recent WHO classification of CNS tumors classifies neoplasms based on their histopathological and molecular characteristics. Modern laboratory techniques, such as matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry, enable the comprehensive metabolic analysis of the tissue sample. MALDI imaging is able to characterize the spatial distribution of a wide array of biomolecules in a sample, in combination with histological features, without sacrificing the tissue integrity. In this review, we first provide an overview of GBM epidemiology, risk, and protective factors, as well as the recent WHO classification of CNS tumors. We then provide an overview of mass spectrometry workflow, with a focus on MALDI imaging, and recent advances in cancer research. Finally, we conclude the review with studies of GBM that utilized MALDI imaging and offer our perspective on future research.

ATX-101, a cell-penetrating protein targeting PCNA, can be safely administered as intravenous infusion in patients and shows clinical activity in a Phase 1 study

Proliferating Cell Nuclear Antigen (PCNA) is a highly conserved protein essential for DNA replication, repair and scaffold functions in the cytosol. Specific inhibition of PCNA in cancer cells is an attractive anti-cancer strategy. ATX-101 is a first-in-class drug targeting PCNA, primarily in cellular stress regulation. Multiple in vivo and in vitro investigations demonstrated anti-cancer activity of ATX-101 in many tumor types and a potentiating effect on the activity of anti-cancer therapies. Healthy cells were less affected. Based on preclinical data, a clinical phase 1 study was initiated. Twenty-five patients with progressive, late-stage solid tumors were treated with weekly ATX-101 infusions at four dose levels (20, 30, 45, 60 mg/m²). ATX-101 showed a favorable safety profile supporting that vital cellular functions are not compromised in healthy cells. Mild and moderate infusion-related reactions were observed in 64% of patients. ATX-101 was quickly cleared from blood with elimination half-lives of less than 30 min at all dose levels, probably due to both, a quick cell penetration and peptide digestion in serum, as demonstrated in vivo. No tumor responses were observed but stable disease was seen in 70% of the efficacy population (n = 20). Further studies have been initiated to provide evidence of efficacy. Trial registration numbers: ANZCTR 375262 and ANZCTR 375319.

PCNA regulates primary metabolism by scaffolding metabolic enzymes

The essential roles of proliferating cell nuclear antigen (PCNA) as a scaffold protein in DNA replication and repair are well established, while its cytosolic roles are less explored. Two metabolic enzymes, alpha-enolase (ENO1) and 6-phosphogluconate dehydrogenase (6PGD), both contain PCNA interacting motifs. Mutation of the PCNA interacting motif APIM in ENO1 (F423A) impaired its binding to PCNA and resulted in reduced cellular levels of ENO1 protein, reduced growth rate, reduced glucose consumption, and reduced activation of AKT. Metabolome and signalome analysis reveal large consequences of impairing the direct interaction between PCNA and ENO1. Metabolites above ENO1 in glycolysis accumulated while lower glycolytic and TCA cycle metabolite pools decreased in the APIM-mutated cells; however, their overall energetic status were similar to parental cells. Treating haematological cancer cells or activated primary monocytes with a PCNA targeting peptide drug containing APIM (ATX-101) also lead to a metabolic shift characterized by reduced glycolytic rate. In addition, we show that ATX-101 treatments reduced the ENO1 - PCNA interaction, the ENO1, GAPDH and 6PGD protein levels, as well as the 6PGD activity. Here we report for the first time that PCNA acts as a scaffold for metabolic enzymes, and thereby act as a direct regulator of primary metabolism.

Glioblastoma Treatment: State-of-the-Art and Future Perspectives

(1) Background: Glioblastoma is the most frequent and lethal primary tumor of the central nervous system. Through many years, research has brought various advances in glioblastoma treatment. At this time, glioblastoma management is based on maximal safe surgical resection, radiotherapy, and chemotherapy with temozolomide. Recently, bevacizumab has been added to the treatment arsenal for the recurrent scenario. Nevertheless, patients with glioblastoma still have a poor prognosis. Therefore, many efforts are being made in different clinical research areas to find a new alternative to improve overall survival, free-progression survival, and life quality in glioblastoma patients. (2) Methods: Our objective is to recap the actual state-of-the-art in glioblastoma treatment, resume the actual research and future perspectives on immunotherapy, as well as the new synthetic molecules and natural compounds that represent potential future therapies at preclinical stages. (3) Conclusions: Despite the great efforts in therapeutic research, glioblastoma management has suffered minimal changes, and the prognosis remains poor. Combined therapeutic strategies and delivery methods, including immunotherapy, synthetic molecules, natural compounds, and glioblastoma stem cell inhibition, may potentiate the standard of care therapy and represent the next step in glioblastoma management research.